The present invention relates to apparatus and methods for optical triangulation measurement of the type employing a measuring beam, and, more particularly, to such apparatus and method in which the position of the measurement beam affects the measurement accuracy.
Optical triangulation systems have been commonly employed for measurement and for inspection applications. For example, one such prior art optical triangulation system is shown in FIG. 1A and is generally designated 10. In the system 10, a workpiece 12 is positioned to receive the light output of a light source 14. The light source 14 is shown as having an Idealized Centerline along which the light source measuring beam 15 is directed. The Idealized measuring beam 15 strikes surface 12S of the workpiece 12 at point A.sub.1 and is then reflected back through focusing lens 16 until it impacts as point A.sub.i on photo detector 18. A reference plane, designated REF, is provided. Point B is defined by the intersection between the reference plane and the Idealized Centerline of the light source 14. The reflection from point B through focusing lens 16 is at impact point B' on photo detector 18. For reasons which will be more fully understood later, impact point B' is taken as the reference or zero point of photo detector 18.
The reflection of the Idealized measuring beam 15 from the surface 12S (from point A.sub.1) at point A.sub.i on photo detector 18 is a measure of the distance between the reference plane and the surface 12S. More particularly, the distance between A.sub.i and B' is related through similar triangles to the distance between B and A.sub.1. The distance A.sub.i to B' (zero point) is simply processed to give the distance d between the referece plane and the surface 12S. The output 19 of the photo detector 18 typically comprises an electrical signal which contains information representative of the distance between A.sub.i and B' (zero point). This output signal is typically received by measurement intelligence 20 which interprets the electrical signal and produces a measurement output signal 22 representative of the distance d.
Although the system 10 of FIG. 1A is successful for many applications, one problem with such system occurs due to drift of the light source 14. More particularly, thermal and other conditions often cause drifting of the light source 14 so that the light source 14 may be positioned with an Actual Centerline which is displaced from the Idealized Centerline. For example, in FIG. 1A, an Actual Centerline is shown in phantom parallel to, but displaced from, the Idealized Centerline wherein the measuring beam 15 impacts surface 12S at point A.sub.2 instead of A.sub.1. It is important to appreciate that such displacement of the light source 14 results in the reflected measuring beam impacting on the photo detector 18 at a point displaced from where it would have inpacted had the light source been positioned on the Idealized Centerline. More particularly, the measuring beam 15 on the phantom Actual Centerline impacts the photo detector 18 at point A.sub.a instead of point A.sub.i. Thus, under these drift conditions, the photo detector output 19 will be incorrect due to the measurement error introduced by the displacement of the light source 14 from the Idealized Centerline position.
Another commonly employed triangulation measurement system is shown in FIG. 1B and is generally designated 25. The system 25 is substantially the same as the system 10 of FIG. 1 so that, whenever possible, like reference numerals are employed to represent like elements. The system 25 is representative of optical triangulation measurement systems in which a photo detector 18 is positioned to receive a portion of the measuring beam 15 which is reflected from the surface 12S of the workpiece 12. The portion of the reflected measuring beam received at photo detector 18 is characterized as being representative of the point of impact of the measuring beam on the workpiece 12. In this connection, as in FIG. 1A, a reference or zero point is typically provided on the photo detector 18. For purposes of clarity, the reference plane is not shown in FIG. 1B. In FIG. 1B, centerline drift is shown wherein the Actual Centerline of the light source 14 is shown displaced, resulting in impact point C.sub.a on the surface 12S, but, due to the apparatus having an Idealized Centerline, the photo detector 18 interprets the point of impact on the surface 12S to be idealized point C.sub.i, resulting in a measurement error.
Accordingly, it is a general object of this invention to provide methods and apparatus of optical triangulation measurement for reducing the measurement error introduced as a result of light source centerline drift.
Another object of the present invention is to provide such method and apparatus wherein the relationship between the Idealized Centerline and the Actual Centerline is monitored.
Another object of the present invention is to provide such method and apparatus which includes translating the light source to reduce the measurement error.
Another object of the present invention is to provide such method and apparatus which includes compensation for the measurement error otherwise introduced by the displacement of the Actual Centerline from the Idealized Centerline.